1. Field of the Invention
The invention relates to a signal processing apparatus for compression encoding a digital video signal.
2. Description of the Related Arts
As represented by a digital VTR (Video Tape Recorder), a data recording and reproducing apparatus for recording a digital video signal and a digital audio signal onto a recording medium and reproducing them from the recording medium has been known. Generally, since a data amount of the digital video signal is very large, the signal is compression encoded and recorded onto the recording medium. In recent years, an MPEG2 (Moving Picture Experts Group 2) system has been known as a standard system for compression encoding.
In the MPEG2, data obtained by predictive coding using motion detection and a DCT (Discrete Cosine Transform) is quantized and compression encoded and, further, compression efficiency is improved by using variable length coding.
In the case of a frame unit, the compression encoding using the DCT is executed. For example, the inputted digital video signal is divided on a block unit basis, a DCT arithmetic operation is executed to the divided blocks, and coefficients obtained by the DCT arithmetic operation are quantized. Quantized representative values are variable length coded and compression encoded. Hitherto, as quantization rounding precision which is used upon quantization in the DCT arithmetic operation, for example, a predetermined method such as method whereby fractions of 5 and over are counted as a unit and the rest is disregarded, method whereby fractions of 6 and over are counted as a unit and the rest is disregarded, method whereby the figures after a predetermined numerical value are omitted, or the like has fixedly been used.
In the case where the rounding precision upon quantization is fixed and the quantization is executed as mentioned above, it is difficult to efficiently compression encode both an image which can be easily quantized and an image which is difficult to be quantized. As a method of dynamically changing the rounding precision upon quantization, for example, a method of changing the quantization rounding precision in dependence on a magnitude of an amplitude of a DCT coefficient as disclosed in JP-A-10-304362, or a method of changing the quantization rounding precision in dependence on a frequency component of the DCT coefficient as disclosed in JP-A-10-304363 has been proposed. However, even if such a method is used, picture quality is deteriorated by predetermined encoding noises.
In recent years, the video signal is generally digitized and handled as a digital video signal, while an analog video signal according to a conventional analog system is still used. In such a case, for example, an analog interface corresponding to the analog video signal is provided for the digital VTR. The analog video signal is inputted to the analog interface, converted into the digital video signal, and processed.
Hitherto, the quantization rounding precision which can be set every quantization scale value is constant for the same quantization scale value both in a digital interface environment and an analog interface environment.
However, the analog interface has various unstable factors which are peculiar to an analog circuit, such as disturbance of waveform characteristics, variation in frequency characteristics, and the like which are caused by factors such as influence of noises, variation in adjustment, and temperature drift. Therefore, in the case of using the analog interface, a problem such that noise components are emphasized, an S/N ratio deteriorates, or the like is caused, so that the rounding precision for obtaining the picture quality that is optimum in the analog interface environment is not always obtained.